Strain gauges for measuring strain in various different structures and materials are well known. Such strain gauges typically utilize various different types of transducers in which a change in resistance or capacitance is indicative of a corresponding change in strain.
Although such electrical strain gauges are generally suitable for measuring strain, those skilled in the art will appreciate that such electrical transducers are not suitable for use in some particular applications. For example, it is generally not desirable to utilize electrical transducers in explosive environments, where it is possible that an electrical spark may initiate an undesirable explosive reaction. Further, in some applications the presence of electricity may undesirably interfere with sensitive electronic equipment and the like. Further, in some applications ambient electrical fields may undesirably effect the performance of such electrical transducers. Further, the electricity associated with such electrical transducers may cause the undesirable generation of heat.
Optical strain sensors are known for eliminating such undesirable characteristics of electrical strain gauges. Such optical strain sensors typically comprise Fabry-Perot interferometers wherein the cavity thereof is disposed along the length of an optical fiber which may either be attached to or embedded within a material or structure for which strain measurement is desired.
However, one problem commonly associated with the use of simple Fabry-Perot optical strain sensors is that no indication of the direction of the strain, i.e., compressive or tensile, is indicated thereby. Contemporary simple Fabry-Perot optical strain sensors provide only an indication of the magnitude of the strain applied thereto and do not provide any indication of absolute strain. As used herein, the term absolute strain indicates a strain measurement with which a direction is associated. Thus, a measurement of absolute strain provides both the magnitude of the strain and an indication as to whether the strain is compressive or tensile in nature.
In an effort to provide a measurement of absolute strain, various different prior art devices have been developed. Such prior art devices utilized dual Fabry-Perot interferometers wherein the signal output of each of the interferometers are in quadrature with one another. Thus, an indication of whether the etalon is decreasing or increasing in length is provided as the Fabry-Perot interferometer experiences either compression or tension. One example of such a prior art dual interferometer strain sensor is provided in U.S. Pat. No. 5,301,001, issued on Apr. 5, 1994 to Murphy et al and entitled EXTRINSIC FIBER OPTIC DISPLACEMENT SENSORS AND DISPLACEMENT SENSING SYSTEMS.
However, as those skilled in the art will appreciate, prior art devices which facilitate the measurement of absolute strain are comparatively complex. Two separate fiber optic signal cables are required. Additionally, two separate optical sensors and their related electronics must also be utilized. The complexity of such devices inherently reduces their reliability and also makes them more difficult to use. This is particularly true since two separate optical fibers must be imbedded, mounted, and/or routed. Such prior art dual interferometer strain sensors are also inherently more expensive, due to the increased number of components thereof.
In view of the foregoing, it is desirable to provide a fiber optic strain sensor which measures both the magnitude and direction of strain applied thereto and which is simple in construction, so as to enhance the reliability and ease of use thereof, while also reducing the cost thereof.